An automatic laser power control device of an optical system may maintain an optical level of an optical signal (which is output from a laser diode of an optical pickup unit) at a constant level. More particularly, the automatic laser power control device may maintain the optical level of the output optical signal at a constant level by compensating for changes in the output optical signal caused by temperature variation in the laser diode.
An automatic laser power control device may include a voltage generator that applies a control voltage to a driving circuit of the laser diode to equalize the optical level with an initially set target optical level. The voltage generator, which includes a digital-to-analog (D/A) converter, may provide the following characteristics.
First, when reading data from a disc, an optical system may generate some particular voltages of the voltage generator and measure optical output characteristics (which are the characteristics of an output of the laser diode) to determine a relationship between the optical output characteristics and the voltages generated by the voltage generator. Thus, an output of the voltage generator should provide a wide range of voltage outputs to determine a wide range of laser diode output characteristics.
Second, a voltage signal corresponding to the optical level of an optical signal and an output voltage of the voltage generator may be applied to an input terminal of a gain controller of the automatic laser power control device of the optical system. A gain controller may amplify a difference in level between the voltage signal and the output voltage by a set voltage gain and may output the result of amplification.
For precise control of the voltage gain, the gain controller may set the voltage gain at a high level. Accordingly, a resolution of the output voltage of the voltage generator (which is applied to the input terminal of the gain controller) may be set to a unit of several mV. Third, a linearity of an output of the voltage generator should be provided.
Accordingly, an output of the voltage generator of the optical system may provide a wide range from a ground voltage to a power supply voltage, and an output voltage of the voltage generator may provide resolution of a unit of several mV and linear characteristics. In general, to provide the above described characteristics, an 8 to 10 bit D/A converter may be used as the voltage generator.
A conventional resistive array D/A converter may provide a small layout size but an output of the converter may have significant nonlinear characteristics caused by external factors such as a mismatch in resistances, an error of the ratio of resistances, and/or a change in a switch resistance.
A signal output from a current mode D/A converter may provide improved linear characteristics. However, an increase in the number of bits of the signal may result in an increased number of unit current cells of the converter. A significant current may be consumed and/or a layout size may be increased.